1. Field
The present invention relates generally to data communication, and more specifically to a novel and improved method and apparatus for utilizing (full or partial) channel state information to provide improved performance for a wireless communication system.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), orthogonal frequency division modulation (OFDM), or some other modulation techniques. OFDM systems can provide high performance for some channel environments.
In a terrestrial communication system (e.g., a cellular system, a broadcast system, a multi-channel multi-point distribution system (MMDS), and others), an RF modulated signal from a transmitter unit may reach a receiver unit via a number of transmission paths. The characteristics of the transmission paths typically vary over time due to a number of factors such as fading and multipath.
To provide diversity against deleterious path effects and improve performance, multiple transmit and receive antennas may be used. If the transmission paths between the transmit and receive antennas are linearly independent (i.e., a transmission on one path is not formed as a linear combination of the transmissions on other paths), which is generally true to some extent, then the likelihood of correctly receiving a transmitted signal increases as the number of antennas increases. Generally, diversity increases and performance improves as the number of transmit and receive antennas increases.
A multiple-input multiple-output (MIMO) communication system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel may be decomposed into NC independent channels, with NCxe2x89xa6min {NT, NR}. Each of the NC independent channels is also referred to as a spatial subchannel of the MIMO channel and corresponds to a dimension. The MIMO system can provide improved performance if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
There is therefore a need in the art for techniques to utilize channel state information (CSI) to take advantage of the additional dimensionalities created by a MIMO system to provide improved system performance.
Aspects of the invention provide techniques to process received signals in a multiple-input multiple-output (MIMO) communication system to recover transmitted signals, and to estimate the characteristics of a MIMO channel. Various receiver processing schemes may be used to derive channel state information (CSI) indicative of the characteristics of the transmission channels used for data transmission. The CSI is then reported back to the transmitter system and used to adjust the signal processing (e.g., coding, modulation, and so on). In this manner, high performance is achieved based on the determined channel conditions.
A specific embodiment of the invention provides a method for transmitting data from a transmitter unit to a receiver unit in a MIMO communication system. In accordance with the method, at the receiver unit, a number of signals are received via a number of receive antennas, with the received signal from each receive antenna comprising a combination of one or more signals transmitted from the transmitter unit. The received signals are processed (e.g., via a channel correlation matrix inversion (CCMI) scheme, an unbiased minimum mean square error (UMMSE) scheme, or some other receiver processing scheme) to derive CSI indicative of characteristics of a number of transmission channels used for data transmission. The CSI is encoded and transmitted back to the transmitter unit. At the transmitter unit, the CSI from the receiver unit is received and data for transmission to the receiver unit is processed based on the received CSI.
The reported CSI may include full CSI or partial CSI. Full CSI includes sufficient full-bandwidth characterization (e.g., the amplitude and phase across the useable bandwidth) of the propagation path between all pairs of transmit and receive antennas. Partial CSI may include, for example, the signal-to-noise-plus-interference (SNR) of the transmission channels. At the transmitter unit, the data for each transmission channel may be coded based on the SNR estimate for the transmission channel, and the coded data for each transmission channel may be modulated in accordance with a modulation scheme selected based on the SNR estimate. For full-CSI processing, the modulation symbols are also pre-processed prior to transmission in accordance with the received CSI.
The invention further provides methods, systems, and apparatus that implement various aspects, embodiments, and features of the invention, as described in further detail below.